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The second generation of chemosensitizers includes analogs of the above-mentioned compounds that were designed to reduce their toxicity. Among them, the (R)-isomer of verapamil, named dexverapamil, lacks its cardiac effects while retaining the ability to inhibit P-gp. Similarly, PSC-833 (valspodar) is a structural analogue of cyclosporin A that lacks its immunosuppressive effects. However, in spite of their improved activity, both compounds are non-selective. For instance, PSC-833 inhibits P-gp and MRP2, as well as other proteins of the ABC superfamily. The clinical value of these agents is also uncertain because, as it will be mentioned later, their substrate specificity for P-gp is similar to the one they show for cytochrome P450, which explains why PSC-833 is involved in pharmacokinetic interactions with most anticancer drugs, requiring dose adjustment and leading to a high interpatient variability.15 It is worth mentioning that it has been suggested that valspodar and cyclosporin may have an anticancer effect independently of their action on MDR.

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